American Sign Language (ASL) is a complete language that uses signs made with the hands, body and facial expressions. Many deaf North Americans use this language but other communication options are available as well. ASL is one of the top five most commonly used languages in the United States. It is possible ASL was developed from its counterpart French Sign Language (FSL).
Dual Language Perspective
American Sign Language and English are two very different languages. In order to facilitate the fast pace of normal speech, ASL abbreviates English in a way that makes comprehending proper grammar difficult. Writing and reading can be difficult if you learn ASL first. Many words are skipped over or assumed so deaf people can keep up with their own thoughts as they translate the information from brain to hand. If a child learns sign first, oftentimes he will think in sign rather than English. Teaching English as a second language and employing those techniques is important to ASL students. It is easy to forget ASL does not make someone a native English speaker.
Isolation in Learning the Language
Children learn language by being bombarded with it all around. On a daily basis any number of people speak to children, and even when not a part of the conversation the child can listen. If a child is not brought up in a deaf home or around many other deaf people, sign language tends to be used only when speaking with the non-hearing person. This limits the access to learning and increasing vocabulary. Deaf people who utilise hearing aids or cochlear implants often develop a well-rounded vocabulary quickly. This puts the student on par with others. Using only ASL can slow the vocabulary of the deaf.
Crutch Keeping Deaf People from Speaking
It is never a guarantee that a deaf person will speak. Some never become comfortable with vocal communication. Some parents feel giving a child ASL as an option keeps him from trying to expand past his comfort zone. If a child can communicate effectively with sign language he may never try to speak. This tends to be a problem for children who attend deaf schools where not speaking is a very viable option. Mainstreaming deaf children to hearing schools often helps encourage them to participate. At that point ASL becomes a hindrance in the respect of "being different," drawing attention to the student and making him uncomfortable.
Inadequate Language Models
If a deaf child is born to hearing parents, the parents are usually playing catch-up. In general it takes five years to become proficient in a new language, especially if you are not immersed in it. This means your child does not have a truly proper and adequate language model. Your child learns by modelling your behaviour, so this is the same as your attempting to teach your child French as a first language when you speak English.